Old Cars--Modern Fuels

In my job as an engineer at Continental Automotive, I work on the development and production of fuel injectors. In Newport News, we design and produce injectors used by auto manufacturers all over the world. Through my experience, I have come to learn about gasoline and the effects it can have, both good and bad, on fuel systems.

In various magazines and web forums there is a lot of discussion about old or classic cars and modern fuels. Many of the sites have good information based on controlled tests. Unfortunately, there also seems to be a lot of anecdotal information unsupported by data. Although I am not a petroleum expert, I thought I would share what I know about the composition of modern fuels and how they can affect our older cars. By "older" I am thinking about cars build prior to the introduction of unleaded fuel in the mid-70's. For those of you that drive more modern cars, some of this information will still be relevant.

Unleaded Fuels

Beginning in 1968 the US enacted laws to control the emissions from automobiles. The first systems simply controlled the emissions from the engine crankcase. By the mid 1970's there were controls on tailpipe emissions: specifically carbon monoxide (CO), oxides of nitrogen (NOx), and unburned fuel (hydrocarbons - HC). These gases were major contributors to the smog and ground level ozone in places like Los Angeles. The CO & HC components are usually the result of incomplete combustion in the engine from a rich mixture or a misfire. The NOx is formed at high combustion temperatures.

The earliest exhaust emission control systems, with air pumps and carburetors tuned to provide lean mixtures, were only moderately effective. The next, and perhaps the greatest step, to improved control of engine emissions was the introduction of an exhaust catalyst. The catalyst promotes chemical reactions that reduce the NOx to nitrogen and oxygen, oxidise the CO to CO2, and burn the HC to produce CO2 and water.

One obstacle to the use of an exhaust catalyst is the tetraethyl lead (TEL) that had, in the past, been used to improve the octane rating of gasoline. The lead in this compound would cover the surface of the catalyst making it ineffective. In addition and on a more general basis, the emission of lead into the environment was unhealthy, so unleaded fuel was introduced. This was the first fuel modification that could have a detrimental effect on some older cars.

The tetraethyl lead also had the effect of lubricating and protecting the valve seats. This effect was crucial for the few engines that did not have hard valve seats. However, even those engines are unlikely to suffer any serious effect when not heavily loaded. In general, the loss of TEL will not caused a problem for over 90% of our old cars. One other consequence of the elimination of TEL was a reduction of octane level. As a result, many engines of the mid 70's had reduced compression ratios. Since that time, fuel blends have been developed that have octane levels at least as high as those previously achieved with TEL.

Alcohol Content

In order to improve the efficiency of modern fuel and emission control systems, it is mandated that gasoline include compounds that contain oxygen. In the US, this function is fulfilled by ethanol. The alcohol also has the benefit of increasing the octane value of the fuel. I won't get into the dubious benefits of burning alcohol in order to reduce our oil dependence. That is another subject altogether. For now, we just have to accept the fact that alcohol is a component of almost all gas sold in the US.

In almost every part of the US, our gasoline contains 10% ethanol; referred to as E10. The inclusion of ethanol in gasoline can have a couple of adverse effects. The rubber compounds used in the fuel systems of old cars were not selected for compatibility with alcohol and the physical properties of those rubbers can deteriorate in the presence of alcohol. Considering that, NOS gaskets and seals could suffer premature failures and may not be well suited to vehicles that are driven. And besides the alcohol compatibility issue, rubber components will degrade with age, regardless of use. Alternatively, any engine system hose or gasket manufactured today is likely to be well suited to modern fuel mixtures. The other adverse effect of alcohol as a fuel is its hygroscopic tendency, it attracts and retains water. This property of the alcohol means that E10 fuel will contain up to 1% water. The presence of water will increase the electrical conductivity of the fuel and can cause the formation of acids. This increased conductivity and acidity will promote galvanic reactions. Getting beyond all the chemistry talk, the net effect is that there will be more corrosion of brass or aluminum within the fuel system. At the current 10% level of alcohol content, this corrosion is not a significant issue. However, there is a proposal to increase to E15 in the US and that level of alcohol could cause problems. As a counterpoint, the oil companies now include anti-corrosive additives in gasoline. No, they are not worried about your fuel system components. They are protecting their gasoline pipelines.

As a side note, one other effect of an E10 fuel mixture is a slight loss of fuel efficiency. The energy content per gallon of alcohol is about 34% lower than typical gasoline. This means an E10 mixture will require slightly more fuel to achieve the same engine output. This slight change in the energy content of the fuel also changes the optimum ratio of fuel to air for proper combustion. Fortunately, the change is so small that changes of carburetor needles or jets are not typically required. As with any other type of gasoline, the spark plug colour can be used to judge if the fuel mixture is lean, stoichiometric (correct) or rich.

Fuel Additives

The oil companies spend a lot of time and money developing combinations of additives to improve the performance of their fuels and oils. We all know about the viscosity modifiers used to make a wide variety of oil blends; 5W-20, 20W-50, etc. The same type of effort is made with fuels. However, instead of trying to control viscosity, the intent is to promote high octane, complete burning, and corrosion control. Incidentally, the leading company in the development of these additive packages for the oil companies is Afton Chemical, headquartered right here in Richmond. (Prior to a recent name change, Afton was Ethyl Corp.)

You have probably seen lots of advertising lately by Shell about their "nitrogen enriched" fuel to clean intake valves. This is not just hype. One of the distinguishing features of the various fuel brands is the level of detergent. (No! Do not pour Tide in your fuel tank!) The purpose of the detergents is to keep the fuel injectors, intake valves, and combustion chambers free of deposits. The use of detergents began in the mid-80.s, coinciding with the widespread introduction of fuel injection systems. Because fuel injectors have very small flow passages, they are particularly sensitive to deposit build-up. For our old cars, the carburetor jets and needles are the sensitive areas.

Although there is a federal standard for minimum fuel detergent content, the quantity of detergents varies by brand. The no-name fuels generally have the least detergent, while the brand name fuels, and especially their high octane blends, have notably higher detergent levels. I drive my 1968 Mini with dual SU's on a daily basis and I always use a high octane brand name fuel. In the past 5 years, I have not had any issues associated with fuel deposits.

In your local auto parts store, there is an aisle full of colorful bottles containing a wide variety of fuel additives. Many of these are purported to boost the octane value of the fuel. In looking at the results of some well controlled tests, none of these additives seems to offer more than 1 or 2 points on a premium grade fuel. And none are a cost effective method for boosting the octane value of low or mid-grade fuel.

On the other hand, some of the fuel system cleaners that you buy at your local auto parts store can provide a more significant benefit than improved octane. These products are formulated to clean the deposits from the intake valves and the combustion chamber. By reducing or eliminating the deposits, the engine will breathe better and the effective compression ratio will be reduced. In extreme cases, those combustion chamber deposits could be hot-spots that cause pre-ignition. Even without an octane boost, the desired result of reduced pinging and improved performance can be achieved. Great!

You can find a huge amount of related content on the internet. As with everything you find on the web, be cautious to distinguish between opinions and facts that are supported by data.